THE 500 RULE
Before we look at camera settings, here’s an important rule to learn – the 500 rule. The earth spins at about 360º every 24 hours. If you look at the stars over time, they will appear to move, but in fact we’re the ones moving. Because we’re taking long exposures, the stars will move during that time and we want them to appear as sharp dots – not trails (unless you’re trying to shoot star trails).
So how long can we expose an image before we get noticeable trails? Well, just divide 500 by the focal length of your lens. The result is the maximum number of seconds you should expose for. For example – for a 14mm lens – 500/14 = gives you around 35 seconds. If you shoot for 30 seconds then you are safe.